1. Field of the Invention
The present invention generally refers to a switching power supply, in particular a power factor corrector (PFC), and more particularly to a circuit for the programmable protection of output over-voltages and an integrated circuit comprising a circuit for the programmable protection against output over-voltages.
2. Description of the Prior Art
A power supply consisting of a full-wave rectifier bridge and a filter capacitor connected immediately downstream produces a non-regulated direct voltage starting from a sinusoid alternating voltage main. The capacitance of the filter capacitor is large enough that, at the leads of the capacitor, there is a relatively small ripple compared to the continuous D.C. level. The rectifier diodes of the bridge therefore will conduct only for a small portion of each half-cycle, given that the instantaneous value of the line voltage is lower than the voltage on the filter capacitor for the majority of the cycle. It follows then that the current drawn from the voltage main consists of a series of sharp pulses whose width is 5–10 times the resulting average value.
These sharp pulses present considerable consequences and shortcomings. On shortcoming is the absorption of sinusoidal current, the current absorbed from the voltage main has much greater peak and effective values and the voltage main is distorted by effect of the pulsed absorption.
In order to mitigate the consequence of sharp pulses, switching techniques are used. These switching techniques include a power factor corrector (PFC), positioned between the rectifier bridge and the filter capacitor. Using a PFC, a nearly sinusoidal current is absorbed from the voltage main, which is in phase with the voltage. This PFC moves the power factor near 1 and reducing the Total Harmonic Distortion (THD).
Usually the switching configuration called boost is used, which generates a direct voltage that is greater than the maximum voltage main peak voltage, typically 400V for systems powered with European or with universal voltage main.
The power factor corrector comprises a control loop which, starting from information relating to the output voltage (Vout), regulates the turn-on or turn-off of a power transistor whose function is substantially that of a switch.
The analysis of the control loop bandwidth shows the presence of a very low crossover frequency, a fact that inevitably leads to definitely slow controller response times.
This characteristic of the loop is however absolutely necessary to prevent the output voltage from being able to follow the sinusoidal shape of the rectified voltage main, no longer supplying, as a consequence, a nearly direct voltage.
Nevertheless, there are situations in which the reduced bandwidth is an important limit that may lead to several components of the circuit being damaged.
As an example, let us presume, that a sudden load variation occurs, such that the current requested passes, for example, from 100% to 10% or less.
For the reason given previously, the controller is not capable of adjusting in a sufficiently short time to the new operating conditions, therefore an over-voltage is generated on the output node which, if reaching too high values, could damage the discrete components of the pre-regulator stage, in particular the power transistor and the filter capacitor.
Accordingly, a need exists to overcome the problems with the prior art and to provide a protection circuit to protect against the output over-voltages